1. Field of the Invention
The present invention concerns an amplifier device comprising at least one operational amplifier, whereby a transformer is upstream from the input of the operational amplifier and the output signal of the operational amplifier or a signal generated from this output signal is fed back again at the input of the operational amplifier via a path with a predetermined resistance.
2. Related Art
Operational amplifiers and amplifier devices with operational amplifiers are widely known in communications engineering, acoustics and sensor technology. In order to achieve an optimal signal transfer given the amplification, on the one hand an impedance adaptation must be effected. What is meant by impedance adaptation is the process of adapting the source impedance and the impedance of a load to one another since the power transfer is therewith maximized and reflections at the load are minimized. On the other hand, the noise factor N should be optimally minimized (also referred to as noise adaptation). This results from the quotient of the input signal-to-noise ratio (input SNR) and the output SNR and therewith indicates the reduction of the SNR. The current noise figure at NF=10·logN is calculated from the noise factor. Since the noise figure appears dependent on the source impedance offered to the operation amplifier, a common optimum for both quality criteria is not necessarily found in the design of the amplifier input circuit.
Two characteristics of the operation amplifier are thereby to be strongly differentiated. What is known as the small signal input resistance reflects the differential resistance between the two inputs of the operation amplifier and is infinitely large in the ideal case. In the impedance adaptation this resistance is adapted to the source impedance (mostly 50 Ohm). This is to be differentiated from what is known as the equivalent noise input resistance, which results from the quotient of the noise voltage density and the noise current density of the equivalent alternate sources of the operation amplifier. This quantity mostly lies in the kΩ range and is not a truly measurable resistance, but rather ultimately renders an adaptation value to other resistances in the system. An internal resistance of the source strongly deviating from this causes a high noise and therewith a high noise figure.
In the simplest known method for impedance adaptation, the input signal of the operation amplifier is connected to ground given a source impedance of 50Ω via a further 50 Ohm resistor, whereupon an adaptation of the small signal input resistance is affected. However, a very large noise figure is generated since in typical amplifiers the equivalent noise input resistance lies in the kΩ range, such that the values deviate from one another by orders of magnitude and the termination resistance also supplies an additional noise contribution.
An amplifier device for simultaneous power and noise adaptation is proposed in the data sheet for operation amplifier CLC425 of the company National Semiconductor Corporation, 2900 Semiconductor Drive, Santa Clara, Calif., 95041 U.S.A (National Semiconductor CLC425 Ultra Low Noise Wideband Op Amp, 8 May 2001, available on the Internet). The adaptation to the source in particular is thereby achieved via a resistance feedback of the amplifier output signal to the amplifier input. This active manner of adaptation avoids the forced adaptation (described above) via the noise-plagued termination resistance at the amplifier input and thus leads to a reduced noise figure of the amplifier. For noise adaptation a transformer is connected upstream from the circuit. The signal inversion necessary for the negative feedback is realized in the proposed arrangement in the signal output path via an active function block (inverter). However, this amplifier device exhibits some disadvantages. Due to the use of the inverted a power loss occurs in the same, As an additional component the inverter also requires a certain space. Finally, as an active element the inverter itself represents a noise source, which further worsens the noise figure.